Semiconductor device manufacturing method and plasma etching apparatus

ABSTRACT

A semiconductor device manufacturing method includes a plasma etching step for etching an etching target film formed on a substrate accommodated in a processing chamber. In the plasma etching step, a processing gas including a gaseous mixture containing predetermined gases is supplied into the processing chamber, and a cycle including a first step in which a flow rate of at least one of the predetermined gases is set to a first value during a first time period and a second step in which the flow rate thereof is set to a second value that is different from the first value during a second time period is repeated consecutively at least three times without removing a plasma. The first time period and the second time period are set to about 1 to 15 seconds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2010-024552 filed on Feb. 5, 2010 and U.S. Provisional Application No.61/310,513 filed on Mar. 4, 2010, the entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device manufacturingmethod and a plasma etching apparatus.

BACKGROUND OF THE INVENTION

A semiconductor device manufacturing process includes a process forplasma-etching various films formed on a substrate, e.g., asemiconductor wafer or the like, provided in a processing chamber of aplasma etching apparatus.

In the plasma etching apparatus, the interior of the processing chamberaccommodating therein a substrate, e.g., a semiconductor wafer or thelike, is set to a depressurized atmosphere of a predetermined pressureand a predetermined processing gas is supplied into the processingchamber. The processing gas is converted to a plasma by a RF (radiofrequency) electric field or the like. By applying the plasma of theprocessing gas to the substrate, various films formed on the substrateare plasma-etched.

As for a plasma processing method using the above plasma etchingapparatus or the like, there has been known a method for etching siliconwithout generating an undercut by forming a nitride film on a surfacewhile temporarily stopping etching by intermittently stopping the supplyof SF₆ gas in a gaseous mixture supplied into a processing chamber for ashort period of time, the SF₆ gas serving to facilitate etching (see,e.g., Japanese Patent Application Publication No. H4-73287).

Along with the trend toward miniaturization of a circuit pattern of asemiconductor device, a pattern size has been reduced from about 56 nmto about 43 nm and further to about 32 nm. For that reason, a patternformed by plasma etching tends to be miniaturized and become increasedin height or depth. Therefore, a technique for uniformly forming suchpattern with high accuracy and high selectivity has been developed.However, due to the trade-off relationship between the selectivity andthe pattern shape, it is difficult to form a pattern, e.g., a thin anddeep hole, a line-and-space pattern having a small width and a tallheight, or the like, with high selectivity.

SUMMARY OF THE INVENTION

In view of the above, the present invention provides a semiconductordevice manufacturing method and a plasma etching apparatus, capable ofuniformly forming a fine pattern with high accuracy and highselectivity.

In accordance with an aspect of the present invention, there is provideda semiconductor device manufacturing method including a plasma etchingstep for etching an etching target film formed on a substrateaccommodated in a processing chamber. In the plasma etching step, aprocessing gas including a gaseous mixture containing a plurality ofpredetermined gases is supplied into the processing chamber, and a cycleincluding a first step in which a flow rate of at least one of thepredetermined gases is set to a first value during a first time periodand a second step in which the flow rate thereof is set to a secondvalue that is different from the first value during a second time periodis repeated consecutively at least three times without extinguishing aplasma generated in the processing chamber. The first time period andthe second time period are set to range from about 1 to 15 seconds. Atotal flow rate of the processing gas in the first step and a total flowrate of the processing gas in the second step are set to besubstantially equal to each other, or a difference between the totalflow rates, if there exists, is set to range within about 10% of thelarger one of the total flow rates. In each of the first and the secondstep, a gas for facilitating etching of the etching target film iscontained in the processing gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become apparentfrom the following description of embodiments, given in conjunction withthe accompanying drawings, in which:

FIG. 1 schematically shows a configuration of a plasma etching apparatusin accordance with an embodiment of the present invention;

FIGS. 2A to 2F explain a semiconductor device manufacturing method inaccordance with an embodiment of the present invention;

FIGS. 3A to 3C respectively provide electron microscope images ofpatterns of a test example 1 and comparative examples 1 and 2;

FIGS. 4 to 13 are graphs showing a result of examination on in-planeuniformity of an etching rate;

FIGS. 14A to 18 are graphs showing temporal variations in plasmaemission intensities in the case of changing processes;

FIG. 19 shows a structure of a semiconductor wafer in accordance with atest example 2;

FIGS. 20A to 20C respectively provide electron microscope images ofpatterns of the test example 2 and comparative examples 3 and 4;

FIG. 21 shows a structure of a semiconductor wafer in accordance with atest example 3; and

FIG. 22 provides an electron microscope image of a pattern of the testexample 3.

DETAILED DESCRIPTION OF THE EMBODIMENT

Embodiments of the present invention will now be described withreference to the accompanying drawings which form a part hereof.

FIG. 1 schematically shows a configuration of a plasma etching apparatus200 in accordance with an embodiment of the present invention. Theplasma etching apparatus 200 includes a processing chamber 1 which isairtightly configured and electrically grounded. The processing chamber1 has a cylindrical shape, and is made of, e.g., aluminum having ananodically oxidized surface.

Disposed in the processing chamber 1 is a mounting table 2 forsupporting thereon a semiconductor wafer W as a target substratehorizontally. The mounting table 2 is made of, e.g., aluminum having ananodically oxidized surface, and serves as a lower electrode. Themounting table 2 is supported by a conductive support 4 via aninsulating plate 3. Further, a focus ring 5 formed of, e.g.,single-crystalline silicon, is disposed on an outer peripheral portionof a top surface of the mounting table 2. Moreover, a cylindrical innerwall member 3 a made of, e.g., quartz or the like, is provided so as tosurround the mounting table 2 and the support 4.

The mounting table 2 is connected to a first RF (radio frequency) powersupply 10 a via a first matching unit 11 a and also connected to asecond RF power supply 10 b via a second matching unit 11 b. The firstRF power supply 10 a serving to generate a plasma supplies, to themounting table 2, an RF power having a predetermined frequency (higherthan or equal to 27 MHz, e.g., about 40 MHz). Further, the second RFpower supply 10 b serving to attract (bias) ions supplies, to themounting table 2, an RF power having a predetermined frequency (lowerthan or equal to 13.56 MHz, e.g., 2 MHz) lower than that of the first RFpower supply 10 a.

Meanwhile, a shower head 16 serving as an upper electrode is providedabove the mounting table 2 so as to face the mounting table 2 inparallel. The shower head 16 and the mounting table 2 serve as a pair ofelectrodes (upper electrode and lower electrode).

An electrostatic chuck 6 for electrostatically attracting and holdingthe semiconductor wafer W is provided on the top surface of the mountingtable 2. The electrostatic chuck 6 includes an insulator 6 b and anelectrode 6 a embedded therein, and the electrode 6 a is connected to aDC power supply 12. The semiconductor wafer W is attracted and held onto the electrostatic chuck 6 by a Coulomb force or the like which isgenerated by a DC voltage applied from the DC power supply 12 to theelectrode 6 a.

A coolant path 4 a is formed inside the support 4 and connected to acoolant inlet line 4 b and a coolant outlet line 4 c. By circulating aproper coolant, e.g., cooling water or the like, through the coolantpath 4 a, the temperature of the support 4 and that of the mountingtable can be controlled to respective predetermined levels. Further, abackside gas supply line 30 for supplying a cold heat transfer gas(backside gas) such as helium gas or the like to the backside of thesemiconductor wafer W is formed so as to penetrate through the mountingtable 2 and the like. With such configuration, the semiconductor wafer Wattracted and held on the top surface of the mounting table 2 throughthe electrostatic chuck 6 can be controlled at a predeterminedtemperature.

The shower head 16 is disposed at a ceiling portion of the processingchamber 1. The shower head 16 includes a main body 16 a and an upperceiling plate 16 b serving as an electrode plate. The shower head 16 issupported at an upper portion of the processing chamber 1 by aninsulating member 45. The main body 16 a is made of a conductivematerial, e.g., aluminum having an anodically oxidized surface, and isconfigured to detachably hold the upper ceiling plate 16 b providedunder the main body 16 a.

A gas diffusion space 16 c is formed inside the main body 16 a. Aplurality of gas through holes 16 d is formed at a bottom portion of themain body 16 a so as to be positioned under the gas diffusion space 16c. Further, gas injection holes 16 e are formed in the upper ceilingplate 16 b so as to extend therethrough in its thickness direction andcommunicate with the gas through holes 16 d. With such configuration, aprocessing gas supplied to the gas diffusion space 16 c is dispersed andsupplied in a shower shape into the processing chamber 1 through the gasthrough holes 16 d and the gas injection holes 16 e. Moreover, a line(not shown) for circulating a coolant is provided in the main body 16 aand the like, so that the shower head 16 can be cooled to a desiredtemperature during a plasma etching process.

A gas inlet port 16 g for introducing the processing gas into the gasdiffusion space 16 c is formed at the main body 16 a. The gas inlet port16 g is connected to one end of a gas supply line 15 a. The other end ofthe gas supply line 15 a is connected to a processing gas supply source15 for supplying a processing gas for plasma etching.

The gas supply line 15 a is provided with a mass flow controller (MFC)15 b and a valve V1 sequentially from its upstream side. Further, a gas,e.g., Ar, O₂, C₄F₈, HBr, NF₃, C₄F₆, CF₄ or the like, serving as aprocessing gas for plasma etching is supplied from the processing gassupply source 15 to the gas diffusion space 16 c through the gas supplyline 15 a. The gas is dispersed and supplied in a shower shape into theprocessing chamber 1 from the gas diffusion space 16 c through the gasthrough holes 16 d and the gas injection holes 16 e.

A variable DC power supply 52 is electrically connected to the showerhead 16 serving as the upper electrode via a low pass filter (LPF) 51.The power supply of the variable DC power supply 52 can be on-offcontrolled by an on/off switch 53. The current and voltage supplied fromthe variable DC power supply 52 and the on/off operation of the on/offswitch 53 are controlled by a control unit 60 to be described later. Aswill be described later, when a plasma is generated in a processingspace by applying to the mounting table 2 the RF powers from the firstRF power supply 10 a and the second RF power supply 10 b, the on/offswitch 53 is turned on by the control unit 60 if necessary and, thus, apredetermined DC voltage is applied to the shower head 16 serving as theupper electrode.

A cylindrical ground conductor 1 a is provided to extend upward from asidewall of the processing chamber 1 beyond higher than the shower head16. The cylindrical ground conductor 1 a has a ceiling plate at itsupper portion.

A gas exhaust port 71 is formed at a bottom portion of the processingchamber 1, and a gas exhaust unit 73 is connected to the gas exhaustport 71 through a gas exhaust line 72. By operating a vacuum pumpincluded in the gas exhaust unit 73, the processing chamber 1 can bedepressurized to a predetermined vacuum level. Further, aloading/unloading port 74 through which the wafer W is transferred isprovided at a sidewall of the processing chamber 1, and a gate valve 75for opening and closing the loading/unloading port 74 is provided at theloading/unloading port 74.

Reference numerals 76 and 77 denote detachable deposition shields. Thedeposition shield 76 is installed along the inner wall of the processingchamber 1, and the deposition shield 77 is provided so as to surroundthe support 4 and the mounting table 2. The deposition shields 76 and 77serve to prevent etching by-products (deposits) from being attached tothe inner wall of the processing chamber 1 and the like.

The entire operation of the plasma etching apparatus having theabove-described configuration is controlled by the control unit 60. Thecontrol unit 60 includes a process controller 61, a user interface 62,and a storage unit 63. The process controller 61 includes a CPU tocontrol various units of the plasma etching apparatus.

The user interface 62 includes a keyboard for inputting commands, adisplay unit for displaying an operation status of the plasma etchingapparatus to allow a process manager to manage the plasma etchingapparatus, and the like.

The storage unit 63 stores therein recipes including control programs(software) for implementing various processes performed in the plasmaetching apparatus under control of the process controller 61, processcondition data and the like. If necessary, a certain recipe is calledfrom the storage unit 63 in accordance with an instruction input throughthe user interface 62 and executed in the process controller 61.Accordingly, a desired process is performed in the plasma etchingapparatus under the control of the process controller 61. Further, therecipes including control programs, process condition data and the likemay be retrieved from a computer-readable storage medium (e.g., a harddisk, a CD, a flexible disk, a semiconductor memory or the like), orretrieved on-line through, e.g., a dedicated line from another availableapparatus whenever necessary.

Hereinafter, a sequence for plasma-etching an insulating film and thelike formed on a semiconductor wafer W by using the plasma etchingapparatus configured as described above will be explained. First, thegate valve 75 opens, and the semiconductor wafer W is loaded from a loadlock chamber (not shown) into the processing chamber 1 by a transferrobot (not shown) through the loading/unloading port 74 to be mounted onthe mounting table 2. Then, the transfer robot is retreated outwardlyfrom the processing chamber 1, and the gate valve 75 is closed.Thereafter, the processing chamber 1 is evacuated via the gas exhaustport 71 by the vacuum pump of the gas exhaust unit 73.

After the inside of the processing chamber 1 is evacuated to apredetermined vacuum level, a predetermined processing gas (etching gas)is supplied from the processing gas supply source 15 into the processingchamber 1. When the inside of the processing chamber 1 is maintained ata predetermined pressure of, e.g., about 4.7 Pa (35 mTorr), an RF powerhaving a frequency of, e.g., 40 MHz, is supplied from the first RF powersupply 10 a to the mounting table 2. Further, an ion attraction (bias)RF power having a frequency of, e.g., 2.0 MHz, is supplied from thesecond RF power supply 10 b to the mounting table 2. At this time, apredetermined DC voltage is applied from the DC power supply 12 to theelectrode 6 a of the electrostatic chuck 6, so that the semiconductorwafer W is attracted and held on the electrostatic chuck 6 by a Coulombforce.

By supplying the RF powers to the mounting table 2 as described above,an electric field is formed between the upper electrode, i.e., theshower head 16 and the lower electrode, i.e., the mounting table 2.Thus, a discharge is generated in the processing space where thesemiconductor wafer W is located. As a result, a plasma of theprocessing gas is generated, and an insulating film and the like formedon the semiconductor wafer W are etched by the plasma of the processinggas. At this time, the on/off switch 53 is turned on if necessary toapply a predetermined DC voltage from the DC power supply 52 to theshower head 16 serving as the upper electrode. In the etching process, acycle including a first step in which a flow rate of at least one of theprocessing gases is set to a first value and a second step in which theflow rate thereof is set to a second value that is different from thefirst value is repeated consecutively at least three times withoutextinguishing the plasma generated in the processing chamber 1. Thefirst and the second step will be described in detail later.

After the above-described etching process is completed, the supply ofthe RF powers, the DC voltage, and the processing gas is stopped, andthe semiconductor wafer W is unloaded from the processing chamber 1 inthe reverse sequence to the above-described sequence.

The following description relates to a semiconductor devicemanufacturing method which is performed by using the plasma etchingapparatus 200 configured as described above in accordance with anembodiment of the present invention. FIGS. 2A to 2F schematically showan example of a structure of a semiconductor wafer W to be patterned byplasma etching in the present embodiment.

As shown in FIG. 2A, a silicon nitride film 102 (thickness of, e.g.,about 20 nm), a silicon oxide film 103 (thickness of, e.g., about 500nm), a carbon film 104 (thickness of, e.g., about 670 nm), a siliconoxide film 105 (thickness of, e.g., about 40 nm), a bottomanti-reflection coating film 106 are formed in that order from thebottom on the surface of a silicon substrate 101. Further, twophotoresist films 107 and 108 patterned in a predetermined shape (inwhich holes having a predetermined inner diameter are formed atpredetermined intervals in the present embodiment) are formed on thebottom anti-reflection coating film 106.

In the present embodiment, the bottom anti-reflection coating film 106and the silicon oxide film 105 are plasma-etched from a state shown inFIG. 2A to a state shown in FIG. 2B. Next, the carbon film 104 isplasma-etched to a state shown in FIG. 2C.

Thereafter, the silicon oxide film 103 is plasma-etched to a state shownin FIG. 2D. Then, the carbon film 104 remaining on the silicon oxidefilm 103 is removed by ashing to a state shown in FIG. 2E. Lastly, thesilicon nitride film 102 is etched to a state shown in FIG. 2F. In thisway, a plurality of hole-shaped patterns penetrating the silicon oxidefilm 103 having a thickness of about 500 nm and the silicon nitride film102 having a thickness of about 20 nm is formed at a predeterminedinterval.

In the present embodiment, the case of plasma-etching the silicon oxidefilm 103 will be described. At this time, as for a processing gas, agaseous mixture of, e.g., a fluorine compound gas, O₂ gas and Ar gas, isused. As for the fluorine compound gas, it is preferable to use, e.g.,C₄F₆ gas. Moreover, in the present embodiment, a cycle including a firststep in which a flow rate of C₄F₆ gas for facilitating etching is set toa first value and a second step in which the flow rate thereof is set toa second value that is different from the first value is repeatedconsecutively at least three times without extinguishing the plasmagenerated in the processing chamber 1.

In this regard, a first time for performing the first step and a secondtime for performing the second step are preferably set to range fromabout 1 to 15 seconds, and more preferably set from about 2.5 to 10seconds. The reason thereof will be described later.

Moreover, the total flow rate of the processing gas in the first stepand that in the second step are preferably set to be substantially equalto each other. If the total flow rates are different, it is preferableto control the difference therebetween to range within about 10% of thelarger one of the total flow rates. In other words, when the total flowrate of the processing gas in the first step is, e.g., about 1000 sccmand larger than that in the second step, the total flow rate of theprocessing gas in the second step is set between about 900 sccm and 1000sccm. Accordingly, the pressure in the processing chamber in the firststep and that in the second step become set to be substantially equal toeach other and, thus, it is possible to suppress a drastic change in thestate of the plasma etching process, which is beyond the pressurevariation range in which an APC (automatic pressure control) valveprovided at the processing chamber is operable.

In each of the first and the second step, a gas for facilitating etchingof an etching target film (silicon oxide film in the present embodiment)is contained in the processing gas. Therefore, in the presentembodiment, a fluorine compound gas is contained in the processing gasin each of the first and the second step. Hence, the decrease in theetching rate can be prevented.

As a test example 1, the silicon oxide film 103 was plasma-etched byusing the plasma etching apparatus shown in FIG. 1 under the followingconditions.

Pressure: 4.7 Pa (35 mTorr)

RF power (H/L): 2000/4000 W

DC voltage: 150 V

Processing gas (first step): C₄F₆/O₂/Ar=60/65/900 sccm

Processing gas (second step): C₄F₆/O₂/Ar=80/65/900 sccm

Processing time: (10 sec in first step+10 sec in second step)×4 (total80 sec) overetch 41%

Temperature (top/sidewall/mounting table): 150/150/60° C.

He pressure at backside of wafer (center/edge): 2000/5300 Pa (15/40Torr)

In the test example 1, a hole pattern having a desired shape (holediameter of about 45 nm) was formed, and a selectivity of a base layerto the silicon nitride film 102 was about 40. The electron microscopeimage of the pattern at this time is shown in FIG. 3A.

As a comparative example 1, plasma etching was performed under the sameconditions as those of the test example 1 except that a flow rate ofC₄F₆ gas was fixed to about 60 sccm without periodic changes thereof andthe processing time was set to about 90 seconds (overetch 50%). As acomparative example 2, plasma etching was performed under the sameconditions as those of the test example 1 except that the flow rate ofC₄F₆ gas was fixed to about 80 sccm without periodic changes thereof andthe processing time was set to about 90 second (overetch 62%). Theelectron microscope images of the patterns of the comparative examples 1and 2 are shown in FIGS. 3B and 3C, respectively.

In the comparative example 1, the pattern shape was improved, whereasthe selectivity of the base layer to the silicon nitride film 102 waslow (about 19.1). In the comparative example 2, the selectivity of thebase layer to the silicon nitride film 102 was high (about 34.9),whereas the permeability was poor and more patterns showed the etch-stopstate.

As described above, in the test example 1, the shape and the selectivityof the pattern were improved compared to the case of performing plasmaetching while fixing the flow rate of C₄F₆ gas.

Next, the in-plane uniformity of the etching rate was examined byperforming plasma etching on a thermal oxide film formed on a blanketwafer in each case of the same conditions of the test example 1 (exceptfor the processing time of about 80 seconds) (test example 1-2), underthe same conditions of the comparative example 1 (except that theprocessing time was about 80 seconds) (comparative example 1-2), andunder the same conditions of the comparative example 2 (except for theprocessing time of about 80 seconds) (comparative example 2-2). Inaddition, the examination was performed while fixing the flow rate ofC₄F₆ gas to about 70 sccm without periodic changes thereof (processingtime of about 80 seconds) (comparative example 3-2).

The results thereof are shown in the graphs of FIGS. 4 to 7 in which thevertical axis indicates an etching rate and the horizontal axisindicates a position in the surface of the wafer. In the graphs,notations ‘’ and ‘◯’ denote a value measured along the X direction anda value measured along the Y direction perpendicular to the X direction,respectively. As shown in FIG. 4, in the test example 1-2, the averageetching rate was about 430.1 nm/min, and the uniformity was about 8.1%.

On the other hand, in the comparative example 1-2 shown in FIG. 5, theaverage etching rate was about 413.5 nm/min, and the uniformity wasabout 17.5%. Further, the tendency that the etching rate was decreasedat the central portion of the wafer and increased at the peripheralportion of the wafer was noticeable. In the comparative example 2-2shown in FIG. 6, the average etching rate was about 141.0 nm/min.However, portions in which the film thickness was not measurable due toa large amount of deposits (parts that were not plotted in FIG. 6) wereobserved between the peripheral portion and the central portion of thewafer. In the comparative example 3-2 shown in FIG. 7, the averageetching rate was increased to about 463.3 nm/min, whereas the uniformitywas decreased to about 11.6%. Moreover, the tendency that the etchingrate was decreased at the central portion of the wafer and increased atthe peripheral portion of the wafer was noticeable.

As set forth above, in the test example 1-2 in which the flow rate ofC₄F₆ gas was changed periodically, the etching rate and the uniformitywere improved compared to the comparative example in which the flow rateof C₄F₆ gas was fixed.

In the test examples 1 and 1-2, the first step and the second step wereperformed respectively for about 10 seconds, and four cycles of thefirst and the second step were changed periodically. However, the firstand the second step may be performed preferably for about 1 to 15seconds, and more preferably for about 2.5 to 10 seconds. The reasonthereof will be described later.

FIG. 8 shows the result of examination on the etching rate and thein-plane uniformity in the case of performing one cycle of the first andthe second step for about 40 seconds in the test example 1-2. FIG. 9shows the result obtained by performing two cycles of the first and thesecond step for about 20 seconds per each step. FIG. 10 shows the resultobtained by performing 8 cycles of the first and the second step forabout 5 seconds per each step. FIG. 11 shows the result obtained byperforming 16 cycles of the first and the second step for about 2.5seconds per each step. FIG. 12 shows the result obtained by performing40 cycles of the first and the second step for about 1 second per eachstep. FIG. 13 shows the result obtained by performing 80 cycles of thefirst and the second step for about 0.5 second per each step.

As shown in FIG. 13, when the first and the second step respectivelywere performed for about 0.5 second, the result substantially the sameas that obtained in the case of continuously supplying C₄F₆ gas at theflow rate of about 70 sccm (comparative example 3-2 (FIG. 7)) wasobtained, and the uniformity of the etching rate was hardly improved. Inthis case, the average etching rate was about 461.7 nm/min, and theuniformity was about 10.6%.

As illustrated in FIG. 12, when the first and the second step wererespectively performed for about 1 second, the uniformity of the etchingrate was improved compared to the case of continuously supplying C₄F₆gas at the flow rate of about 70 sccm (comparative example 3-2 (FIG.7)). In this case, the average etching rate was about 454.5 nm/min, andthe uniformity was about 9.1%.

As shown in FIG. 11, when the first and the second step wererespectively performed for about 2.5 seconds (the average etching rateof about 446.8 nm/min, the uniformity of about 8.6%) and when the firstand the second step were respectively performed for about 5 seconds asshown in FIG. 10 (the average etching rate of about 447.3 nm/min, theuniformity of about 7.2%), the improvement of the uniformity of theetching rate was gradually increased.

However, when the first and the second step were respectively performedfor a longer period of time, e.g., about 20 seconds, compared to thetest example 1-2 in which the first and the second step wererespectively performed for about 10 seconds, the etching rate and theuniformity were decreased compared to the case of continuously supplyingC₄F₆ gas at the flow rate of about 70 sccm (comparative example 3-2(FIG. 7)), as shown in FIG. 9. In this case, the average etching ratewas about 364.7 nm/min, and the uniformity was about 27.2%. In FIG. 9,parts that are not plotted indicate portions in which the film thicknesswas not measurable due to a large amount of deposits.

Even when the first and the second step were respectively performed forabout 40 seconds as shown in FIG. 8, the etching rate and the uniformitywere decreased compared to the case of continuously supplying C₄F₆ gasat the flow rate of about 70 sccm (comparative example 3-2 (FIG. 7)).

The above result shows that the time for performing the first and thesecond step may be set preferably to range from about 1 to 15 seconds,and more preferably from about 2.5 to 10 seconds. The reason that thedesired result is obtained by setting the processing time of the firstand the second step as described above is considered because atransition state in which the plasma state changes slightly occursduring the plasma etching.

FIGS. 14A and 14B show results of examinations on temporal variations ofthe plasma state in the case of changing the gas flow rates. Here, thehorizontal axis and the vertical axis represent elapsed time and lightemission intensity, respectively. At this time, the plasma was generatedunder the following conditions.

Pressure: 4.0 Pa (30 mTorr)

RF power (H/L): 500/150 W

Processing gas (first step): HBr/Cl₂/NF₃=160/20/20 sccm

Processing gas (second step): HBr/Cl₂/NF₃=140/20/40 sccm

The curved lines in FIGS. 14A and 14B indicate, from the top, the lightemission intensities of CO and SiCl having a wavelength of about 226 nm,N₂ and NH having a wavelength of about 337 nm, and SiF, Cl²⁺ and SiNhaving a wavelength of about 440 nm. As can be seen from FIG. 14A, whenthe first step was changed to the second step by operating a valveprovided outside the processing chamber (by increasing the flow rate ofNF₃ and decreasing the flow rate of HBr), the plasma state started tochange after about 3 seconds and became stable after about 10 seconds.In other words, in that case, the transition state occurred for about 7seconds.

Further, when the second step was changed to the first step (bydecreasing the flow rate of NF₃ and increasing the flow rate of HBr),the plasma state started to change after about 3 seconds and becamestable after about 7 seconds, as can be seen from FIG. 14B. In otherwords, the state became stable in the shorter period of time compared tothe case shown in FIG. 14A. The volume of the processing chamber wasabout 68 liters.

In the case where the first step is changed to the second step and thetransition state is generated for about seconds, if each of the firstand the second step is performed for a short period of time of about 5seconds or less, the transition state is generated for most of theprocessing time, whereas the plasma state may not reach a steady state.If the first and the second step are performed for about 8 seconds, thetransition state is generated for most of the processing time, and theplasma state can be changed to a substantially steady state. Thus, it isconsidered that the above-described effects are obtained by performingeach of the first and the second step for about 8 seconds between about1 to 15 seconds.

FIG. 15 shows the result of measuring the plasma emission intensity inthe case of setting the plasma generating conditions as follows.

Pressure: 4.7 Pa (35 mTorr)

RF power (H/L): 2000/4000 W

Processing gas (first step (10 sec)): C₄F₆/O₂/Ar=60/65/200 sccm

Processing gas (second step (10 sec)): C₄F₆/O₂/Ar=80/65/200 sccm

Here, the emission intensity of CF having a wavelength of about 250 to270 nm is shown in FIG. 15. FIG. 16 shows the light emission intensitymeasured in the case of setting the time for performing each of thefirst and the second step to about 5 seconds under the above-describedconditions; FIG. 17 shows the light emission intensity measured in thecase of increasing the flow rate of Ar gas to about 900 sccm; and FIG.18 shows the light emission intensity measured in the case of increasingthe pressure to about 9.4 Pa (70 mTorr). Moreover, the volume of theprocessing chamber was about 68 liters.

As can be seen from FIGS. 15 to 18, the variation range of the lightemission intensity is decreased by increasing the flow rate of Ar gasand is increased by increasing the pressure. However, the temporallength of the transition state is hardly affected.

Hereinafter, a test example 2 will be described. In the test example 2,a line-and-space pattern was formed by plasma-etching a carbon filmhaving a thickness of about 600 nm. In a semiconductor wafer used in thetest example 2, as shown in FIG. 19, a carbon film 121 having athickness of about 600 nm was formed on a thermal oxide film 120 havinga thickness of about 1 μm, and a silicon oxide film (SiO₂ film) 122having a thickness of about 60 nm and a bottom anti-reflection coatingfilm 123 having a thickness of about 30 nm were formed thereon.

Then, a photoresist 124 having a thickness of about 100 nm and patternedin a predetermined shape was formed on the bottom anti-reflectioncoating film 123. In the test example 2, the bottom anti-reflectioncoating film 123 and the silicon oxide film 122 were etched by using thephotoresist 124 as a mask and, then, the carbon film 121 wasplasma-etched while using the silicon oxide film 122 as a mask.

The carbon film 121 was plasma-etched under the following conditions.

Pressure: 0.67 Pa (5 mTorr)

RF power (H/L): 500/500 W

Processing gas (first step): HBr/O₂=40/40 sccm

Processing gas (second step): HBr/O₂=0/80 sccm

Processing time: (11 sec in first step+11 sec in second step)×4 (total88 seconds)

Temperature (top/sidewall/mounting table): 100/80/40° C.

He pressure at backside of wafer (center/edge): 1330/1330 Pa (10/10Torr)

In the test example 2, a line-and-space pattern having a predeterminedshape was formed by plasma-etching the carbon film 121 having athickness of about 600 nm while ensuring the selectivity to the siliconoxide film 122 serving as a mask layer. The electron microscope image ofthe pattern of the test example 2 is shown in FIG. 20A.

As a comparative example 4, the same plasma etching was performed underthe same conditions as the test example 2 except that the flow rate ofthe processing gas of HBr/O₂ was fixed to about 40/40 sccm. As a result,the etching was stopped in the middle of the processing and was notcompleted. The electron microscope image of the pattern of thecomparative example 4 is shown in FIG. 20B.

As a comparative example 5, the same plasma etching was performed underthe same conditions as the test example 2 except that the flow rate ofthe processing gas of O₂ was fixed to about 80 sccm. As a result, thestate of the mask was not maintained due to the insufficient selectivityto the silicon oxide film 122 serving as a mask layer, and a criticaldimension (CD) of the carbon film 121 was reduced. The electronmicroscope image of the pattern of the comparative example 5 is shown inFIG. 20C.

Hereinafter, a test example 3 will be described. A hole pattern wasformed on an amorphous silicon film having a thickness of about 400 nmpositioned under a carbon film having a thickness of about 300 nm. In asemiconductor wafer used in the test example 3, as shown in FIG. 21, acarbon film 132 having a thickness of about 300 nm was formed on anamorphous silicon film 131 having a thickness of about 400 nm, and asilicon oxide (SiO₂) film 133 having a thickness of about 60 nm and abottom anti-reflection coating film 134 were formed thereon.

Then, a photoresist 135 having a thickness of about 100 nm and patternedin a predetermined shape was formed on the bottom anti-reflectioncoating film 134. In the test example 3, the bottom anti-reflectioncoating film 134 and the silicon oxide film 133 were etched while usingthe photoresist 135 as a mask and, then, the carbon film 132 was etchedwhile using the silicon oxide film 133 as a mask. Thereafter, theamorphous silicon film 131 was plasma-etched.

The amorphous silicon film 131 was plasma-etched under the followingconditions.

Pressure: 16.0 Pa (120 mTorr)

RF power (H/L): 2500/1300 W

Processing gas (first step): NF₃/HBr/O₂=0/300/20 sccm

Processing gas (second step): NF₃/HBr/O₂=5/300/20 sccm

Processing time: (10 sec in first step+10 sec in second step)×3 (total60 sec)

Temperature (top/sidewall/mounting table): 100/80/80° C.

He pressure at backside of wafer (center/edge): 1330/1330 Pa (10/10Torr)

In the test example 3, a hole pattern having a predetermined shape wasformed by plasma-etching the amorphous silicon film 131 having thethickness of about 400 nm. The electron microscope image of the patternof the test example 3 is shown in FIG. 22.

In accordance with the present embodiment, it is possible to provide asemiconductor device manufacturing method and a plasma etchingapparatus, capable of uniformly forming a fine pattern with highaccuracy and high selectivity.

While the invention has been shown and described with respect to theembodiments, it will be understood by those skilled in the art thatvarious changes and modification may be made without departing from thescope of the invention as defined in the following claims.

1. A semiconductor device manufacturing method comprising: a plasmaetching step for etching an etching target film formed on a substrateaccommodated in a processing chamber, wherein in the plasma etchingstep, a processing gas including a gaseous mixture containing aplurality of predetermined gases is supplied into the processingchamber, and a cycle including a first step in which a flow rate of atleast one of the predetermined gases is set to a first value during afirst time period and a second step in which the flow rate thereof isset to a second value that is different from the first value during asecond time period is repeated consecutively at least three timeswithout extinguishing a plasma generated in the processing chamber; thefirst time period and the second time period are set to range from about1 to 15 seconds; a total flow rate of the processing gas in the firststep and a total flow rate of the processing gas in the second step areset to be substantially equal to each other, or a difference between thetotal flow rates, if there exists, is set to range within about 10% ofthe larger one of the total flow rates; and a gas for facilitatingetching of the etching target film is contained in the processing gas ineach of the first and the second step.
 2. The method of claim 1, whereinthe first time period and the second time period are set to range fromabout 2.5 to 10 seconds.
 3. The method of claim 1, wherein the firsttime period and the second time period are set to be equal to eachother.
 4. The method of claim 1, wherein the etching target film is asilicon oxide film; the processing gas contains at least a fluorinecompound gas; and a flow rate of the fluorine compound gas is set to thefirst value in the first step and to the second value in the secondstep.
 5. The method of claim 4, wherein the fluorine compound gas isC₄F₆ gas.
 6. The method of claim 4, wherein the processing gas containsO₂ gas and Ar gas.
 7. The method of claim 1, wherein the etching targetfilm is an amorphous silicon film; the processing gas contains at leastNF₃ gas, HBr gas and O₂ gas; and a flow rate of NF₃ gas is set to thefirst value in the first step and to the second value in the secondstep.
 8. The method of claim 1, wherein the etching target film is acarbon film; the processing gas contains at least HBr gas and O₂ gas;and flow rates of HBr gas and O₂ gas are set to the first value in thefirst step and to the second value in the second step.
 9. A plasmaetching apparatus comprising: a processing chamber for etching anetching target film formed on a substrate accommodated therein; aprocessing gas supply unit for supplying into the processing chamber aprocessing gas including a plurality of predetermined gases; a plasmagenerating unit for converting the processing gas to a plasma; and acontrol unit for controlling a plasma etching process of thesemiconductor device manufacturing method described in claim 1 to beperformed in the processing chamber.